The present invention relates to a method of feedback-controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine, and more particularly to a method of this kind which is adaped to control the air-fuel ratio when the vehicle is running in high altitudes.
An air-fuel ratio feedback control method for internal combustion engines is widely known, e.g. from Japanese Provisional Patent Publication (Kokai) No. 60-1343 owned by the assignee of the present application, which controls the air air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine by the use of a coefficient variable in response to the output of an exhaust gas ingredient concentration sensor arranged in the exhaust system of the engine during operation in an predetermined air-fuel ratio feedback control region.
This known method comprises determining whether the engine is operating in the feedback control region or in a region other than the former region, comparing the output of the exhaust gas ingredient concentration sensor with a predetermined reference value when the engine is operating in the feedback control region, and changing the value of the coefficient in accordance with the result of the comparison, so as to bring the air-fuel ratio of the air-fuel mixture to a desired value.
In the above method, the predetermined reference value is constant irrespective of atmospheric pressure of a place at which the engine is operating. Therefore, in high altitudes, atmospheric pressure becomes lower to decrease the exhaust pressure of the engine, so that the charging efficiency increases, which causes the amount of air drawn into the cylinders to increase relative to the amount of fuel drawn thereinto whereby the air-fuel ratio of the air-fuel mixture becomes leaner, which results in an increased amount of emission of NOx. Further, in high altitudes the air density is low and hence the output of the engine tends to be insufficient. Therefore, the driver is likely to depress the accelerator pedal more deeply, so that the frequency of high load operation of the engine with the throttle valve opening being large can be increased to cause an increase in the combustion temperature of the air-fuel mixture, resulting in that the tendency toward the increased amount of emission of NOx is further intensified.